Sterilize jar (lid as well) in boiling water and dry, making sure there is no remaining moisture.
Fillet herring, making sure all bones are removed.

In the bottom of the jar spoon chopped lemons and spring onions. On top, alternate a layer of herring fillets and a layer of garlic and thyme. Pour lemon juice, vinegar and olive oil over the layers. Close the jar tight and shake to make sure liquid has reached the bottom. Refrigerate overnight to set before opening the jar. You can store renga in the fridge for up to a week.